Magnetic particle spectroscopy

Magnetic particle spectroscopy is a novel measurement method which arose closely related with the introduction of Magnetic Particle Imaging. In MPS – alternatively called Magnetization Response Spectroscopy [50] – a sinusoidal signal of frequency f_exc and sufficiently large amplitude is applied to the MNP sample. Due to the non-linearity of the magnetization curve, the detection signal recorded with a single or a gradiometric induction coil contains odd higher harmonics. Superimposing a static background field causes the appearance of even harmonics. As a consequence of the MNP dynamic magnetic properties, the harmonics spectrum also depends on the excitation frequency.

MPS is a standard method in order to optimize the signal strength and resolution in Magnetic Particle Imaging (MPI) and it combines static and dynamic magnetic properties of MNPs. Current MPS systems operate at an excitation frequency of 25 kHz, matching the drive frequency of most MPI systems. A multivariate MPS systems allowing measurements of the MPS spectra for various excitation frequencies in the range 600 Hz-10 kHz, amplitudes and static background fields providing important information on nanoparticle properties has been constructed. An extension of the excitation frequency range up to the order of 100 kHz is in progress.

Currently, MPS spectra are generally analysed using the Langevin function model extended by a Debye pre-factor accounting for the MNP dynamic magnetic properties. This model is, however, only a rough approximation which does not correctly describe the phase of the harmonics.

Commercial iron oxides magnetic nanoparticle systems such as Resovist® (Schering AG) are very established as contrast agent for Magnetic Resonance Imaging (MRI) and is used in many MPI studies. However, for MPI the behaviour of these magnetic nanoparticle systems is not fully evaluated. Therefore, the Institute of Medical Engineering as part of the University of Luebeck and the Institute of Electrical Measurements and Fundamental Electrical Engineering at the TU Braunschweig work on the development of magnetic particle spectrometers (MPS) to evaluate the performance of various commercially available MRI contrast agents as well as specifically synthesized MPI tracers.

An MPS can be interpreted as a zero-dimensional MPI scanner consisting of one drive field coil and one receive coil. In contrast to a multi-dimensional MPI scanner, no spatial encoding is applied. The derivative of the particle magnetization is measured by the receive coil. To calculate the characteristics of the nanoparticles, physical models are applied. The current spectrometer setup at the University of Luebeck provides a maximal oscillating field strength of 40 mT at 25 kHz and a maximal offset field strength of 40 mT as well. As the field profile is not homogeneous enough, particles at different places experience a slightly different field strength. Additionally, the voltage induced in the receive coil is dependent on its sensitivity profile, which leads to a spatial dependent amplitude of the particle response. As the physical models are very sensitive against the drive field and amplitude disparities, the homogeneity of the current setup is not high enough. To provide valid data, the magnetic fields and sensitivity profiles have to provide relative errors below 1 ‰ which is a challenging task for the field generator and the geometric as well as the electric design. The MPS system will be used to characterize the magnetic nanoparticle systems in the NanoMag project.

Project Partners

Project partners


	This project has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 604448